Band pass tuning circuits



July 24,1934. DALPAYRAT 1,967,570

BAND PASS TUNING CIRCUITS Filed May 1O 1930 I V F 0 INVENTOR D r Henri F'mngoi; Dahmqral A BY i 2 V HTTQRII EY i: applicable-to receivers for-wireless telephony or bandeiiectively and the other frequencies of the;

Patented July 24, 1934 wBAND: PASS TUNING CIRCUITS nor tenants lPatents Gcrporatiom-New York,

'9 Claims.

-l l1e inventiondescribed-relates to electric tun-v 1 ing: circuits, and more particularly to circuits for.

receiving alternating cur-rent signals, including a band of frequencies. The invention isespecially similar wireless transmission where the signal is ransmitted lay-modulation of aca-rrier wave and .is composed. of a definiteband: of frequencies.

1 whenselectivity; or the degreeoiseparation :'-13--i ofa radiosignatof .onecarrier frequency from :undesiredwaves: of :different carrier frequencies,

-: isaeccmpl-ishecl:byrtherusual method: employing a. tuned; circuit orseveral circuits, the frequency.

x response characteristic of the receiver is rather .1215; peaked,- since there is vonly-one'frequency at which the capacitive and inductive iieactances of the circuit. arejbalanced. At;- any other .f-requency there .will' be an unbalanced reactance, which cuts down the response -.to:such frequency. Thus, in receivstring a modulated wave oi the" character referred 'LOJCOIIIDIlSGdEOf a-=band of individual frequencies, r

1 such a systemwill receive.- one frequencyof the -.-band'.less efiectivelyor not at all, with a resulting signal distortion.

In ordento overcome this: disadvantage, :it has been-proposed, in the past,-.-toconstruct'a-=receiviing-circuit'in such a-manner that it is, so to speak," 1 transparent for a-olefinitewbandof'irequencies, insteadnof a single frequency or in other-words, that its. frequency response characteristic-as. sumes a.-shape.as near=as-possible to rectangular form, instead-of the. markedly peaked-shape corresponding=.to. a single tuned circuit. Such-circui-tse are-we1l known in the art and "commonly Qreferred totas electric-filters. ThCYiJCOIlSiSt.-:S' sentially of a definite number oi individuaL-units =or circuits,thus'constituting achain of reactance units connected:alternatelymin :multiples and sin series. As is: wellknown," the "sharpness. of: a the.

-..accordance with myinventi -leastatdouble effect as t r frequency response characteristic. of such a' filter -increases with the number of vfilterv units-,- or in other words,.the cut oil frequencies at both ends of the bandwbecome' more. marked and the curve 7 assumes ashaper :more equal to: :theideal" rectangular form= as the' 'number; Ofnrthfl individual .of a-receiver, consists in theme of a number of variable condensers-placed in difierent :tuned cirwcuits,v such ast'the tuned circuitscoupling successive amplifying stages and detun-ingthese :circuits in respect to eachptherwin sucha-.;manner= that a by aha-superposition 'aof their: individual peaked wens-1m siren era-Frans a-zicnrssir iZQ-FFAEGE -resonance characteristics, the resultant action of I abroad hand characteristic: in-the output circuit -o:t .the receiver isobtained.

The disadvantage of the latter means to create ceiver, elies in the necessity.

of a greatnumber of tuningelementsat eachstage, or a combination of staggered tuning eleme nts at -several stages whereby the construction of the set is unnecessarily complicated: or else theenergy amplifica-' *tionat-eachoi" the staggered stages of: tuning does not reach its maximum amount, since the maximum iramplification -will only occur when exact tuning is satisfactory at each stage.

Furthermore, it has-been. found, :whenusing filter circuitas referred toabove, in the. an-

tenna, or between" the antennaand the receiver,

it is-rnecessary to use mor e than twozunits for such filter, in"order:to obtain apractically rec- .tangular rshaped response curve such as is re-:

quired'in-"breadcasting where only a restricted channel of. frequencies has -beenallotted to the individual.broadcasting .stations, and where it sible, without interference;

a 1 frequency 1 response the" carrier waves of of the individual sig- 111161channels with"62511-013161. This-ca1ls f-or characteristic which sharplyrcuts off withtthe limit frequencies of a .-radiorbroadcasting signalling band,- a which it. has been-found, has .to'have a widthofvatleast10,000

cycles, inorder-to insure faithful reproduction of "speech and music.

invention-refers to anew, improvedband pass :filter: system, especiallysuitable forradio reception, P which with a -small" number of filter .-units,-. preerably two, provides a considerably increased eiiect as to the sharpness of the cut off frequencies at the extremities of the band.

:In other-words, I provide a band pass filter, in

on, which will have at 0: its 'efiectiveness or sharpness of: the frequency cutoff. To obtain nsists broadly in that pass the filter a second-time and-isionly then applied to the re- "ceiveritself. By'thus utilizing the filter,

speak: twice, the 1 effect is having .the' double number so to obtained of a-filter of circuit units and accordingly presenting an -.improved 1 response frequencies.

is further object achieve characteristic as to .thesharpnessofthe cut off d by my invention, is 110 that of stable regeneration sumcient to increase the signal to a considerable extent over the full wavelength range of tuning and, at the same time, retaining the band pass filter action suitable for the proper reception of signals.

A further object is to provide a system in which the desired signal is transferred and even accentuated, while the undesired signals are suppressed or minimized.

Another object is to provide two tuned circuits with extremely small capacity coupling by means of which band pass filter action is secured over the desired wavelength range.

This and further objects of my invention, as

well as other desirable features of same, will be explained at length in the following detailed description taken with reference to the accompanying drawing. I wish it to be understood that the drawing and the description are intended to be illustrative only of my broad inventive idea and that the invention is subject to many modifications coming within the broad and most comprehensive terms, as expressed in the appended claims.

Figure 1 shows a radio receiving circuit with the feature of the invention embodied in it.

Figure 2 is a circuit similar to Figure 1, in a simplified arrangement.

Figure 3 illustrates the frequency response characteristic of a single tuned circuit and of complex circuits or filters as used according to the invention.

Figure 4 is a schematic diagram illustrating the broad idea underlying the invention.

Referring more particularly to Figure 1, I have shown a radio receiving system comprising an antenna-ground system 1, 3 for picking up a modulated radio wave such as a broadcast wave, or any other signal comprised of a definite band of frequencies. The antenna circuit includes an inductance 2, which is coupled to the input inductance 3 for applying the received waves to the receiver. Inductance 3 is connected to the anode 6 and the screen grid 7 of an ordinary screen grid vacuum tube 5, this tube thus being connected in reverse sense, in respect to the antenna or input circuit. 8 is an anode battery for supplying the space discharge current of the tube. A further inductance coil 15 is connected between the control grid 9 and the filament 10 of the tube 5. I have shown furthermore a band pass filter for extending the frequency response range consisting of two multiple tuned circuits, each comprising an inductance 4 and 13, and a variable condenser 11 and 12 respectively. Both these circuits are coupled by a series condenser 14 of comparatively small capacity. The variable plate systems of the condensers 11 and 12 are coupled mechanically, so

1 that they may be controlled by a single knob in the same way as in the case of a single tuned circuit, whereby the operation of the receiver is by no means more complicated than the operation of an ordinary receiver without band pass filter. I have found that the specific type of filter, as just described, is simple and cheap in design and gives increased results as to the sharpness of the cut off in respect to frequencies 7 outside the receiving band, but Iwish it to be understood that any kind of band pass filter may be incorporated in this new circuit, in accordance with the broad spirit of my invention. The inductance 15 is coupled to the grid and filament of a further vacuum tube 17, which is provided with anode battery 18 and output terminals 19.

This vacuum tube 1'7 may be a further amplifying tube or a rectifying tube, depending on the specific use of the receiver.

The operation of the circuit as described, is as follows: The high frequency input energy collooted by the antenna is transferred across the coupling coils 2-, 3, to the input of the-filter, and

after passing the filter circuit,'is applied across the coupling coils 13, 15, partly to the grid of the tube 5 and partly to the grid of the succeeding tube 17. Thus, the energy applied to the tube 5 is amplified and retransferred in a kind of feedback through the anode circuit of tube 5 to the filter, which is thus passed for a second time. The amplified energy, after passing the filter is then directed to the succeeding tube 17. Thus, the filter circuit is traversed twice in succession by the input currents the second time in an amplified form and the effect is obtained of a filter circuit possessing an increased number of circuit units and a correspondingly extended width and improved sharpness of the cut off frequencies from frequencies outside the receiving band.

Considering a signal of a frequency to which the filter is tuned, it will be readily seen that 'such a signal is not only amplified during the passage through the tube 5, but on account of the feedback arrangement there is also regenerative action by means of which the signal is considerably increased above the normal amplification values. I have found by experiments, however, that such regeneration is quite stable and does not result easily in oscillations, especially if a resistance such as 16 is included in the grid return circuit.

Another feature that will become apparent, is that a signal coming in at a frequency different from that to which the filter is tuned, will pass through tube 5 in such a direction as to have the minimum effect on the output circuit. Thus, this current will be applied to the plate 6, which is properly screened, and will exercise only extremely small effect on the grid 9 of the same tube.

Referring to Figure 3, I have shown a frequency response characterictic A for a single tuned circuit of a natural frequency equal to in, consisting of a single inductance coil and a single capacity. This curve illustrates the well-known very peaked characteristic of such a circuit.

A curve of this nature is obtained by plotting as ordinates the amount of current or voltage passing through the filter at different applied frequencies. Thus, if the applied frequency is varied through a, range b, the response of the output circuit will vary in accordance with curve A, reaching a maximum at the value in, which is called the resonant frequency.- If, however, two filter circuits similar to those of Figures 1 or 2 are coupled together by a small capacity or by a loose inductive coupling; then although these two circuits are individually tuned to the same resonance point in, their combined effect will be to widen the range of tuning; thus, instead of curve A, curve B is obtained. Assuming that the frequency band of the message to be received is equal to b, it will be seen that the filter according to curve A would pass only a portion of the frequencies of the message and that more than half of the side frequencies will be out 01f since the range of maximum energy of this filter marked 22' in the diagram is considerif another station outside (if the -'rarig',e b ----with"=some wavelength*within the r ange f b is sending astrong-signal, this strong signal'would be" passed through "such a: filter nnd wduld pro *duce undesirable "effects in the receiving system.

-I have illustrated by curve C'the filter! action which may -be expected by=utilizing 'the system' of connections in accordance with-my invention;

"scale "approximating the scale o'f -the other two 'I have shown this curveto'aireduced ordinate filter curves for the purpose, of' illustration 'Ih 'actual efiect' of passing the :message" through thesame filtertwicegwith" an intermediate regenerm 'tion' bytube- 5-, is to raise the peak values'- to very much higher amplitudes "while-*-1educing, at- -the same time, the fringingfrequencies outsi'de' of action.

the range b. proaching very closely tdidealissecured. "A -sim ilar effect could, of 'course 'be obtainedby iitiliz ing 'filters similar to the on described in con-" -nection with Figures 1 and 2 between several p nfifiilTfor-i hesvvaicuuimtubes. stages of amplification in a multi-stage set but necessarily the connections would--be very 'eom "plicated and the"arrangement wouldrequire ver -carefultuning adjustment. :On the other-hand; the system'- ofmy invention, presents a sir'nple -circuit,-such as'antenna circuit, and at O the In' this way, 'a filter' curve apand convenient way of==securing effective fllter In-"Figure 41- have shown a'schematicdiagram which illustrates" 'th'e'invention' further as to its broadest spirit. I have shown at I the input output circuit, such as an amplifier or any other translating device. The input energy supplied to the input circuit I is passed through a filter F and afterwards diverted into two directions, one part being passed on to the output circuit 0 and the other part being fed back, preferably through an amplifier A and re-applied to the filter, through which the amplified energy passes a second time and is then applied to the output circuit 0. B is a means for preventing the primary input energy from directly passing through the shunt path to the output circuit and thus forcing it through the filter. In the examples shown in Figure 1 and Figure 2, the operation of the devices A and B is obtained by the use of a vacuum tube, preferably a screen grid tube. On account of the screening action of the screen grid to which the input circuit is connected, practically no amount of energy is passed directly from the input through the vacuum tube 5 to the output circuit without passing the filter. It may, however, be understood that in place of a vacuum tube, any kind of a device which exhibits unilateral electric conductivity may be provided for this purpose and that furthermore, it is not absolutely necessary to employ amplification of the feedback energy such as is the case, according to the circuits of Figures 1 and 2, owing to the use of a three-electrode vacuum tube which, besides acting as a unilateral resistance device, also performs an amplification action, as is well understood.

In Figure 2, I have shown a modified circuit arrangement embodying my invention. This figure differs from the arrangement of Figure 1, in that the multiple tuned circuits constituting the filter are coupled inductively by means of coils 20 and 21, in place of a capacitive coupling by the condenser 14, in accordance with Figure 1. Furzthermore;.theiinput= inductance coil :4 'of' the .filter is zcouplednwithxthe antenna self inductance 2 1 -'directly rand "theacondenser 12,:iorming part of ibut* the .filter,nserves. at thee-same: time as a coupling -means of the avacuum tubesfi and 17. According v 'to this-arrangement, .the-inputenergypasses directly' fromsxtheraantenna totthe tuned circuit comprised ofi-the coil l, condenser 11 and primary 20 of the coupling transformer. From this circuit that-energy passes through the :other. tuned circuit 'I l comprised "of the secondary .21-of .the coupling itrarrsformenand'variable condenser :12. From where:partbfwtheenergy will be fed back through he.screenwgriditubeifi topassathei tuned circuits 4,

zsxsamezmanner.asrdescribed inreferen'ce to :Figure :1 ,tanimprovedsband pass filter action is secured. :AtL1'G,=;I haveshownsawariable resistanceintthe :sfe'edbackspath ivihereby :the :amount or feedback .iaenergyrrnay'be adjusted to a suitable :value and;

the circuit prevented from producing self+oscillatrons... .Ill'l. thercircuitstshown, vthis. resistance performs-the further function oflsupplyingz gridibias ofzmyfinuention, itiwill be.:.r.eadily. seen 1 to ,those skilled .infathe 1 art; .zthat :further imodifications of henparticular..ifiltersrand their coupling tothe zregenerative tuhezmay be .used :without :depart 4.2ingfromztherscope:ofmy invention astdefined in ztheaappendedmlaims.

f'zrfrequency-zband of zainiodulated r signal wave, an

input circuit, an output circuit, a band pass filter arranged between said input circuit and said output circuit designed to pass said transmission band, a return circuit from said output circuit to said input circuit including a unilateral conducting device for returning part of the signal energy to said input circuit and passing it through said filter repeatedly.

2. In a receiving system for receiving the transmission frequency band of a modulated signal;

wave, an input circuit, an output circuit, a band pass filter arranged between said input circuit and said output circuit designed to pass said transmission band and having control means for gradually shifting the receiving wave band along the 5.

frequency scale, and a return path from said output circuit to said input circuit including a vacuum tube having its input connected with said output circuit and having its output connected with said input circuit. 3

3. In a receiving system for receiving the transmission band of a modulated signal wave, an input circuit, an output circuit, a three-electrode vacuum tube having its input connected with said output circuit and having its output COIl-r nected with said input circuit, and a band pass filter arranged between said input circuit and said output circuit designed to pass said signal wave.

4. In a system for receiving the transmission band of a modulated signal wave, an input circuit,;

an output circuit, a screen grid vacuum tube having anode, cathode, control grid and screen grid electrode, said input circuit being connected to the anode and cathode of said vacuum tube, and

said output circuit being connected to the cathode i 131;;20, ,2lsan'd'll2 .a second :timewhereby, .in: the i J filthoughi'Iz'havelillustrated two. preferred forms:v

' nected to the anode and cathode of said tube and said output circuit being connected to the control grid and cathode of said tube, a band pass filter also arranged between said input circuit and said output.

circuit for passing said signal wave. 1 Y

6. In a system for receiving the transmission band of a modulated signal Wave, awave receiving input circuit, an output circuit, a vacuum tube having anode, cathode, controlg-rid and screen grid electrodes, said input circuit being connected to the anode and cathode of said tube, and said output circuit being connected to the control grid and cathode of saidtube, a band pass filter comprising two mutually coupled multiple tuned circuits, said filter being coupled to said input circuit and said output circuits, respectively, and passing said signal wave.

7. In a system for receiving the transmission bandof a modulated signal Wave, a wave receiv-. ing input circuit, an output circuit, a vacuum tube having anode, cathode, control grid, and screen grid electrodes, said input circuit being connected to the anode and cathode of said tube, and said output circuit being connected to the control grid and cathode of said tube, a two unit band pass filter comprising two multiple tuned circuits coupled serially by a single capacity and including control means for gradually shifting the transmission wave band along the frequency scale, said band pass filter being electrically connected between said input circuit and said output circuit.

connected to the anode and cathode of said tube and said output circuit being connected to the control grid and cathode, of said tube, a two unit band pass filter comprising two multiple tuned circuits consisting each of an inductance coil and a variable capacity in parallel and being coupled in series by a capacity, one of said multiple tuned circuits being in coupling relationship with said input circuit and the other one of said multiple tuned circuits being in coupling relationship with:

said output circuit, said variable condensers of said filter units being operated by a single control means for gradually shifting the transmission wave band of said filter along the frequency scale.

9. In a system for receiving the transmission wave band of a modulated signal wave, a wave collecting input circuit, an output circuit, and a twounit band pass filter comprising two multiple tuned circuits consisting each of an inductance coil and a variable capacity in parallel and being; coupled with each other in series by acapacity of comparatively small value, one of, said inductance coils being in coupling relationship with said input circuit, and the other of said inductance coils being in coupling relationship with said output circuit, and a single control means for said variable condensers for gradually shifting the transmission wave band of said filter along the 'frequency scale. a p g H HENRI FRANQOIS DALPAYRAT. 

